Process and compositions for the disinfection of waters

ABSTRACT

A process for killing microorganisms and controlling biofouling in high chlorine demand waters comprises mixing two components, one of which is an oxidant and the other an ammonium salt, and adding the mixture immediately to the aqueous system to be treated.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 10/226,927, filed Aug. 23, 2002 now U.S. Pat. No. 7,067,063, which is a divisional of application Ser. No. 09/023,984, filed Feb. 13, 1998, now U.S. Pat. No. 6,478,973, which is a continuation of application Ser. No. 08/639,708, filed Apr. 29, 1996, now abandoned, which is a continuation of application Ser. No. 08/272,276, filed Jul. 8, 1994, now abandoned, which is a continuation of application Ser. No. 07/892,533, filed Jun. 1, 1992, now abandoned.

BACKGROUND OF THE INVENTION

Biological fouling of circulating water is a known and well documented problem Several factors contribute to the problem and govern its extent: water temperature; water pH; organic and inorganic nutrients either from air drawn into the system or from materials naturally occurring in the water or continuously supplied during plant operation; aerobic/anaerobic conditions; the presence/absence of sunlight, etc.

Algae, fungi, bacteria, as well as other simple life forms are found in circulating water. The types of microorganisms and the extent of microbial growth depend on the water source and on the other factors.

Biological growth in circulating water can foul pipelines, accelerate corrosion, attack wood, decrease heat transfer, plug filters, cause imperfections in paper sheets; decompose sizing mixtures, and cause many other process interferences.

Oxidizing biocides including chlorine gas, hypochlorous acid, bromine and other oxidizing biocides are widely used in recirculating water.

“Chlorine demand” is defined as the quantity of chlorine that is reduced or otherwise transformed to inert forms of chlorine by substances in the water; and standard methods have been established for measuring it. In this specification and claims “chlorine demand” is as measured by procedures outlined in “Standard Methods for the examination of water and waste water,”, 16th edition, Methods §409, pages 316-319. The methods are based on applying a specific dose of chlorine to the medium and measuring the residual chlorine left after a given contact time. Chlorine-consuming substances include ammonia and amino derivatives; sulfides, cyanides, oxidizable cations, pulp lignins, starch, sugars, oil, water treatment additives like scale, and corrosion inhibitors.

Microbial growth in the water and in biofilms contribute to the chlorine demand of the water and to the chlorine demand of the system to be treated. Oxidizing biocides were found to be ineffective in waters containing a high chlorine demand, including heavy slimes. Non-oxidizing biocides are usually recommended for such waters.

Chlorination of water having a high content of ammonia or other. ammino-derivatives results in the formation of chloramines. Chloramines are described as poor biocides relative to hypochlorous or hypobromous acid. According to literature, chloramines are slow to react and may be more persistent in water systems (The NALCO water handbook 1988, PCT/US 89/102730 21.6.1989, Great Lakes Chem. Corp. Wat. Sci. Tech. 20 No 11/12, pp. 385-39, 1988, by M. D. Sobsey et al., National Academy of Science, 1980, Drinking Water and Health, Vol. 2, National Academy Press, Washington, D.C.).

Chloramination of drinking water occurs when chlorine reacts with small amounts of ammonia either present in or added to the water.

Traditional chloramination occurs with the addition of free chlorine to the total amount of water for reaction with small amounts of ammonia present in the water, or added to the water in known amounts. Only one reference describes the use of pre-formed monochloramine for the post-disinfection of drinking water (J. Beck et al., Aqua I, 25-33, 1986). In this work, chloramines were formed by mixing ammonium sulfate and hypochlorite solution at a concentration of 1000 ppm; pH was adjusted to 7.5 before the point of dosage to avoid carbonate precipitation.

Chloramines were used to control aftergrowth and biofouling in the surface seawater reverse osmosis plants (Desalination 74, 51-67 (1989) and European Patent Application No. 90108872.4, Nov. 5, 1990, for Du Pont de Nemours and Company). This patent claims the use of chloramine to inhibit regrowth following dechlorination in liquid process streams containing chlorine degradable organic material, that when in degraded form provides energy and carbon source that is assimilable by microorganisms. The chlorinate for the process was made in situ by adding NH₃ gas, NH₄OH, NH₄Cl or (NH₄)₂SO₄. The sources of chlorine were Cl₂ gas, NaOCl, Ca(OCl)₂ and electrolytically generated chlorine.

Chloramines formed in situ during chlorination of cooling water containing ammonia, is considered to have no biocidal effect in the treatment of cooling towers, since chloramines are quickly stripped due to their high volatility [G. Holz Warth et al., Water Res. 18(1), 1421-1427 (1984)].

The disinfection of highly turbid waste water using chlorine had improved when ammonia was added to the waste water (in situ), when longer contact times were allowed [Ataei Khalil Z. et al.; Proc. Annu. Conf. Am. Water Works Assoc., 1988 (Pt. 2), pp. 1763-1770].

Ammonium bromide was not mentioned as a possible source for chloramines. The usual sources are ammonia, ammonium chloride and ammonium sulfate.

Its is a purpose of the invention to provide a process and compositions for killing microorganisms and inhibiting biofouling in waters, especially in cooling waters and aqueous systems having a high chlorine demand waters, and more especially in cooling waters and aqueous systems having a high chlorine demand.

It is another purpose of the invention to provide such a process and compositions that have a high biocidal effect and a high initial rate of kill in high chlorine demand waters.

It is further purpose of the invention to provide such a process and compositions the biocidal effect and the properties of which are constant and predetermined.

Other purposes and advantages of the invention will become apparent as the description proceeds.

SUMMARY OF THE INVENTION

The process according to the invention comprises mixing two components, one of which is an oxidant, preferably an active chlorine donor and still more preferably sodium hypochlorite, and the other, an ammonium salt, preferably chosen among halides, sulfates and nitrates, and adding the biocidal concentrate immediately to the aqueous system to be treated. The frequency, duration and concentration should be determined in each individual case so as to be sufficient to control biofouling.

Preferably, the two ingredients are mixed in a specific order, and specifically the oxidant is added to a solution of the ammonium salt. In a preferred form of the invention, the oxidant is NaOCl and is slowly added to a well-mixed solution of the ammonium salt diluted in the range of 0.01% to 2% equimolar to chlorine, preferably until a final concentration of chlorine in the mixture has reached 0.01-1% as chlorine. Either batch or continuous formation of the biocidal stock solution is effective.

The biocidal mixture was found to be more effective than other oxidizing biocides (such as chlorine or bromine) whenever the demand in the water system exceeds 1.8 ppm Cl₂ out of 2.0 ppm Cl₂ within 60 minutes. The percentage of ingredients are given as weight percent.

The mole ratio N/Cl is preferably 1:1. An excess of N may be used.

The temperature of the water to which the mixture is added, can be in the region of 10-60° C. The temperature of the solution of ammonium salt should be 10-30° C. when NaOCl is added. The pH is controlled by the concentration of the NaOCl solution; preferably the pH range should be 8.0-12.6. The active ingredient was effective at pH 7 and at pH 8. Some decrease in efficacy was noted at pH 9.

The frequency and duration of treatment and concentrations of active ingredient needed in order to maintain good control of biofouling should be determined in each individual case. However, good control was achieved at a level of 3 mg/l as chlorine (4.2 kg NH₄Br for 1000 m³).

The biocidal mixture is very effective for shock treatment of fouled systems, even in cases where the water demand is low, and enables the effective daily use of oxidizing biocides. A level of 9 mg/l (as chlorine) is sufficient to clean a fouled system.

In preferred forms of the invention the mixture is formed and fed either batchwise or continuously by any suitable means, such as by a liquid metering pump or by gravity.

The invention comprises the solutions prepared as described above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following are non-limitative examples of possible applications of the process:

-   Recirculating cool water -   Brewery pasteurizer -   Air washer -   Evaporative cooling water -   Scrubbers -   Pond and lagoon water -   Closed water-cooling systems -   Food plant disinfection -   Bleaching—pulp and paper -   etc.

The process according to the invention is compatible with other water treatment chemicals, corrosion and scale inhibitors, etc.

EXAMPLE 1 Efficacy in Buffer at pH-7.5 Against Pseudomonas sp.

Dosage: 1 ppm as Cl₂;

Chlorine demand: 0.1 ppm out of 1 ppm within 20 minutes

NH₄Br+NaOCl: stock concentration: 1000 ppm as Cl₂

Preparation of stock solution: NH₄Br was dissolved in de-ionized water (2761 ppm). NaOCl (2000 ppm as Cl₂) was quickly added dropwise to the ammonium bromide solution while stirring the mixture. The stock solution was used immediately.

TABLE I Survivors (cfu/ml) after time (minutes) Biocide 1 5 10 20 NaOCl 4 × 10⁶ 1 × 10⁵ 4 × 10² 0 NaOCl + NaBr 5 × 10³ 4 × 10² 4 × 10  0 (1:1) NH₄Br + NaOCl 6 × 10⁶ 2 × 10⁶ 5 × 10³ 0 (1:1) Control — — — 9 × 10⁶ Results in Table I indicate higher rates of kill for NaOBr and NaOCl as compared to NH₄Br + NaOCl in water with low demand for chlorine.

EXAMPLE 2 Efficacy of Ammonium Bromide at Various pHs

Examined MO: bacillus mycoides

Dose: 2 ppm as Cl₂

NH₄Br+NaOCl: molar ratio 1:1; stock concentration: 0.5%;

NH₄Br+NaOCl was either pre-mixed or added in situ to the buffer.

Demand: 1.8 ppm out of 2 ppm of Cl₂ within 60 minutes.

TABLE II Survivors: cfu/ml pH-7.0 pH-8.0 pH-9.0 Treatment 60 m 180 m 24 h 60 m 180 m 24 h 60 m 180 m 24 h NH₄Br + NaOCl 7 × 10⁴ 2 × 10⁴ 2 × 10² 2 × 10⁵ 9 × 10⁴ 2 × 10² 1 × 10⁵ 9 × 10⁴ 1 × 10⁴ premix NH₄Br + NaOCl 2 × 10⁵ 6 × 10⁴ 2 × 10⁴ 1 × 10⁵ 7 × 10⁴ 1 × 10⁴ 2 × 10⁵ 1 × 10⁵ 1 × 10⁵ in situ NaOCl 1 × 10⁵ 1 × 10⁵ 1 × 10⁵ 3 × 10⁵ 1 × 10⁵ 1 × 10⁵ 1 × 10⁵ 1 × 10⁵ 1 × 10⁵ NaOCl + NaBr 1:1 2 × 10⁵ 1 × 10⁵ 1 × 10⁵ 3 × 10⁵ 2 × 10⁵ 2 × 10⁵ 2 × 10⁵ 1 × 10⁵ 1 × 10⁵ non-treated — 3 × 10⁶ 5 × 10⁵ 3 × 10⁶ — 1 × 10⁵ 5 × 10⁵ — 4 × 10⁵ Table II shows that pre-mixed (NH₄Br + NaOCl) a higher rate of kill as compared to either NaOCl or NaOBr, as the demand for chlorine increases. Efficacy was slightly impaired at pH from 8.0 to 9.0.

EXAMPLE 3 Efficacy of NH₄Cl+NaOCl in Water Taken from a Citrus Juice Evaporator: Comparison to Non-Oxidizing Biocides

Water demand: higher than 30 ppm of Cl₂ (out of 30 ppm Cl₂) within 60 minutes.

Concentration of stock solution (NH₄Cl+NaOCl): 1000 ppm.

Algicol II is a quaternary ammonium salt.

TABLE III dose Survivors (cfu/ml) after time (hours) Biocide mg/l 1 24 7 days Kathon 30 2 × 10⁶ 7 × 10² 7 × 10² MBT 30 1 × 10⁶ 1 × 10³ 8 × 10² Algicol II 100  5 × 10⁶ 4 × 10⁶ 9 × 10⁶ NH₄Cl + NaOCl 30 0 0 0 Control — 6 × 10⁶ 3 × 10⁷ 1 × 10⁷ Results in Table III indicate that a mixture of NH₄Cl + NaOCl was more effective than 3 non-oxidizing biocides in water with high demand for chlorine.

EXAMPLE 4 Efficacy of Oxidizing and Non-Oxidizing Biocides is a Starch Sizing Mixture (Paper Industry)

Efficacy was measured at 60° C.

NH₄Br +NaOCl: stock concentration 0.1%.

Dosage: 30 ppm active ingredient.

Incubation temperature 60° C.

TABLE IV Surviving MO_(β) (cfu/ml) after time (hours) Biocide 4 28 48 72 MBT 9 × 10⁴ 4 × 10⁴ 2 × 10⁴ 2 × 10⁴ NaOCl + NaBr 4 × 10³ 6 × 10⁵ 3 × 10⁵ 3 × 10⁶ NaOCl 4 × 10³ 2 × 10⁶ 3 × 10⁶ 3 × 10⁶ NaOCl + NH₄Br 2 × 10  3 0 0 MIT 2 × 10⁴ 2 × 10³ 2 × 10³ 1 × 10³ DBNPA 3 × 10⁴ 2 × 10⁵ 2 × 10⁵ 1 × 10⁵ Control 1 × 10⁵ 3 × 10⁵ 8 × 10⁵ 7 × 10⁵ Results in Table IV prove that a mixture of NH₄Br + NaOCl is more effective than other oxidizing and non-oxidizing biocides in a high demand medium.

EXAMPLE 5 Kinetics of Kill of Various Mixtures of Ammonium Salts Mixed with NaOCl in Water from a Citrus Juice Evaporator

Dosage; 30 ppm as Cl₂

Demand: Higher to 30 ppm out of 30 ppm of Cl₂ during 10 minutes.

Stock concentration of NH₄X+NaOCl: 0.1% as Cl₂

TABLE V residue as Cl₂ Surviving MOs after time free (total) after (minutes); total aerobic time (minutes) (total anaerobic) cfu/ml Biocide 10 20 60 10 20 60 NH₄Cl + NaOCl 5(10) 1(2) 0(0) 5 × 10²(4 × 10²) 3 × 10²(2 × 10²) 1 × 10²(3) NH₄Br + NaOCl 0(0) 0(0) 0(0) 4 × 10²(4 × 10²) 5 × 10²(4 × 10²) 5 × 10²(2 × 10²) NH₄NO₃ + NaOCl 6(9) — 0(0) 4 × 10²(2 × 10) 4 × 10²(7) 8 × 10(2) NaOCl 0(0) 0(0) 0(0) 2 × 10⁵(8 × 10⁴) 2 × 10⁷(3 × 10⁶) 1 × 10⁷(3 × 10) NaOCl + NaBr 0(0) 0(0) 0(0) 2 × 10⁷(4 × 10⁶) 1 × 10⁷(3 × 10⁶) 2 × 10⁷(2 × 10⁶) CONTROL — — — — — 2 × 10⁷(6 × 10⁶) Results in Table V show that mixtures of ammonium salts and NaOCl are effective in controlling aerobic and anaerobic microorganisms in water with high demand for chlorine. Control was achieved within 10 minutes. Under these conditions both NaOCl and NaOBr are impaired by the media. The mixture of NH₄Br + NaOCl did not leave a measurable residue after 10 minutes, yet it was very effective in reducing viable counts within 10 minutes.

EXAMPLE 6 Efficacy of Oxidizing Biocides in Water Taken from a Paper Mill (Thick Stock of Pulp Slurry)

Dose: 15 mg as Cl₂

Microorganisms were incubated at 37° C. (total aerobic viable counts).

Concentration of stock solution: 0.1% as Cl₂.

TABLE VI counts Viable cfu/ml after time (minutes) Biocide 30 70 25 hours DBNPA 2 × 10³ 6 × 10  8 × 10  NaOCl 6 × 10³ 2 × 10³ 1 × 10⁵ NaOCl + NaBr 1 × 10⁴ 5 × 10³ 2 × 10⁶ NH₄Br + NaOCl 1 0 0 CONTROL 7 × 10⁶ — 1 × 10⁷ Results in Table VI prove higher efficacy for NH₄Br + NaOCl as compared to other oxidizing biocides in this heavily loaded water.

EXAMPLE 7 Efficacy of a Series of Biocides in Domestic Waste Containing a High Concentration of Amines

Contact time: 10 minutes

Dose: 60 ppm as Cl₂

Incubation temperature; 270° C.

Stock concentration: 0.2%

N as NH₃: 50 mg/1; pH: 6.10.

TABLE VII Residue Surviving MOs (cfu/ml) free (total) as Cl₂ after 10 minutes Biocide after 10 minutes aerob. anaer. fungi NaOCl 0.2 (1.5) 9 × 10² 3 × 10³ 2 × 10 NH₄Cl + NaOCl 3.0 (9.0) 2 × 10² 4 × 10² 0 DBNPA 0 (0) 2 × 10⁶ 5 × 10⁵ 0 Control — 5 × 10⁷ 1 × 10⁶ 2 × 10 Results in Table VII prove that in the presence of a high NH₃ concentration, NaOCl was less effective than pre-mixed NH₄Cl + NaOCl in controlling microbial growth (in water with high demand for Cl₂); good control was measured after 10 minutes.

EXAMPLE 8 Efficacy of Oxidizing Biocides in Domestic Waste

Non-treated domestic waste roughly filtered; contact time: 10 minutes.

Stock concentration: 0.5% as Cl₂.

Dose: 20 ppm as Cl₂

TABLE VIII residue as Cl₂ ppm Fecal coli total count treatment free (total) cfu/ml cfu/ml NaOCl 0 (0) 5 × 10⁵ 6 × 10⁶ NaOCl + NaBr (1:1) 0 (0) 3 × 10⁵ 7 × 10⁶ NaOCl + (NH₄)₂SO₄ (1:1) 0.05 (0.7)  3 × 10³ 3 × 10⁴ non-disinfected 5 × 10⁵ 1 × 10⁷ Results in Table VIII prove that pre-mixing (NH₄)₂SO₄ with NaOCl resulted in lower viable counts of both fecal coli and total count.

In waste water with high organic load, this disinfection method was superior to disinfecting with either NaOCl or NaOBr.

EXAMPLE 9 Efficacy of Biocides in the Presence of Anti-Scale and Corrosion Inhibiting Treatment (CWC)

Stock concentration: 0.6% as Cl₂

Efficacy against pseudomonas sp.

CWC: 100 mg/1

pH: 9.0

Contact time: 5 hours

TABLE IX dose Survivors Survivors in treatment mg/l in buffer CWC (100 mg/l) DBNPA 4 0 4 × 10⁶ BCDMH 4 0 3 × 10⁵ NH₄Br + NaOCl 1 0 1 × 10³ 5 0 2 × 10  Results in Table IX prove that in the presence of scale and corrosion inhibitors, efficacy of various biocides was impaired to such an extent that much higher dosages of biocides had to be fed in order to maintain good control. The mixture of NH₄Br + NaOCl was less impaired by CWC and established good microbial and algeal control even in the presence of CWC.

EXAMPLE 10 Pre-Mixing Versus in situ Addition of Ammonium Salt and NaOCl

Water from a corn-processing plant; high demand for Cl₂.

Dose: 12 ppm

NH₄Cl+NaOCl: stock concentration: 1%

NH₄Br+NaOCl: stock concentration: 0.5%

Stock solutions were formed at pH 14.0; 7.0, 4.0 and in water.

For in situ addition: both NH₄X and NaOCl were dissolved at the appropriate pH.

TABLE X Survivors cfu/ml (total count after time in minutes) buffer pH-14.0 pH-7.0 pH-4.0 H₂O Treatment 60 180 60 180 60 180 60 180 NH₄Br + NaOCl 1 × 10⁶ 8 × 10⁵ 2 × 10⁵ 1 × 10⁶ 1 × 10⁶ 1 × 10⁶ 6 × 10⁴ 1 × 10⁴ NH₄Cl + NaOCl 1 × 10⁵ 1 × 10⁵ 6 × 10⁴ 6 × 10³ 7 × 10⁵ 1 × 10⁶ 5 × 10⁴ 7 × 10³ NH₄Br + NaOCl 1 × 10⁶ 2 × 10⁶ — — 2 × 10⁶ 2 × 10⁶ 5 × 10⁶ — in situ NH₄Cl + NaOCl 8 × 10⁵ 2 × 10⁵ — — 1 × 10⁶ 1 × 10⁶ 2 × 10⁶ 2 × 10⁶ in situ NaOCl — — — — — — 2 × 10⁶ 2 × 10⁶ NaOCl + NaBr — — — — — — 3 × 10⁶ 3 × 10⁶ Control 4 × 10⁶ — Results in Table X prove that the efficacy exhibited by mixtures of NH₄X + NaOCl depend on the pH and on the mode of formation of the stock mixture. In situ addition of the two ingredients to water resulted in lower efficacy at any of the examined pH's.

Stock mixture of NH₄Br+NaOCl was more effective when prepared in water than when prepared in buffer at pH 7.0. When the stock solution was prepared at a high or at a low pH it was less effective.

EXAMPLE 11 Dependence of Efficacy of Mixtures of NH₄Br+NaOCl on the Concentrations of Stock Solution

Work was carried out in industrial waste water.

Stock concentrates were prepared in buffer at pH 7.00.

Biocidal dose: 4 ppm as Cl₂.

TABLE XI Survivors cfu/ml treatment total count after time (minutes) NH₄Br + NaOCl 10 60 180   2% 6 × 10⁴ 1 × 10⁴ 2 × 10³   1% 2 × 10⁵ 3 × 10⁴ 3 × 10⁴  0.5% 7 × 10⁴ 2 × 10⁴ 4 × 10³ 0.01% 2 × 10⁶ — 2 × 10⁶ in situ 5 × 10⁵ 3 × 10⁵ 5 × 10⁵ NaOCl 9 × 10⁵ — 7 × 10⁵ Control 1 × 10⁶ — — Results in Table XI prove that the efficacy exhibited by the mixtures correlated with the concentration of stock solutions. The highest efficacy was measured with a stock concentration equal to at 0.5% as Cl₂. Similar trends were obtained when the stock solutions were prepared in water rather than in buffer (see Table X) (The high efficacy measured in buffer at a level of 2% as Cl₂ results from the higher pH of this mixture.)

FIELD EXPERIMENTS EXAMPLE I Cooling Tower 1

-   -   Cooling tower; contained volume 1000 m³     -   Circulation rate 500 m³/h     -   Scale and corrosion inhibitor: CWC: 100 mg/1

The tower was controlled on low level (0.6-1.2 kg/day) of BCDMH feed. Use of BCDMH was effective as long as make-ups were softened in ion-exchangers.

When CWC (100 mg/1 of phosphonate) replaced the use of ion-exchangers, much higher dosages of BCDMH (4-5 kg/day) did not suffice to prevent biofouling and growth of algae.

The system was shock-fed with NH₄Br+NaOCl. Overall dosage: 75 liters NaOCl (10%) 12.6 kg NH₄Br. The mire was fed during 1.5 hours. This shock treatment cleaned the system

A slug dose of 25 liters NaOCl (10% as Cl₂) (+4.2 kg NH₄Br) was then fed to the cooling tower once in two to three days. The cooling tower remained clean, with no apparent growth of biofilm or algae. A measurable residue of 0.6-0.4 ppm (as total chlorine) was measured in the water 24 and 48 hours after feeding the mixture.

EXAMPLE II Cooling Tower 2

-   -   Corn processing plant.     -   Contained volume: 20 m³     -   Circulation rate: 300 m³/h     -   pH: 7.5-8.0     -   Water temperature: 36° C.-57° C.

This tower was treated with BCDMH (1.50-2.26 kg/day) daily. Due to a very high organic load in the water, growth of biofilm was very fast. Treatment with BCDMH was effective in controlling the daily grown films, but was not effective against heavy slimes which covered the cooling tower.

A daily feed of 3 liters NaOCl (7% as Cl₂), mixed with 0.35 kg NH₄Br controlled the daily newly formed biofilm as well as the slime and algae growth covering the cooling tower, and left a clean cooling system after three weeks of daily treatment avoiding the need for shock treatment.

EXAMPLE III Starch Sizing Mixture

Paper mill, starch sizing

Contained volume: 20 m³

Flow rate: 8.33 m³/h. (6% starch in H₂O),

pH about 8.0

Temperature: 50-70° C.

Sizing mixture is recirculated in a size press through a filter (80 microns). Circulation rate: 6 m³/h. The sizing mixture had been previously treated with NaOCl (10% as Cl₂), which was fed every 8 hours (30 liters per portion). With this treatment, filters had to be washed once every two hours.

Use of NaOCl was replaced by the use of a mixture of NH₄Br+NaOCl (stock concentration 0.5% as Cl₂).

Feeding of NaOCl (13 liters of 10% as Cl₂) and NH₄Br (2.5 kg) three times a day (every eight hours) kept the filters in the size press clean; the treatment with NH₄Br+NaOCl was compatible with a blue dye added to the sizing mixture, and did not bleach the blue starch, unlike NaOCl.

A number of embodiments of the invention have been described for purposes of illustration, but it will be understood that they are not limitative and that the invention can be carried out by persons skilled in the art with many modifications, variations and adaptation, without departing from its spirit and from the scope of the appended claim. 

1. A process for killing microorganisms and controlling biofouling in high chlorine demand water, which comprises mixing a solution of sodium hypochlorite and a solution of ammonium bromide, in a molar ratio calculated as N/Cl of at least 1:1, to form a biocidal mixture, the concentration of said mixture being 0.1% to 1.0% as Cl₂, and adding said biocidal mixture to said high chlorine demand water, said mixture having a pH of 8.0 to 12.5, said mixture being characterized in that it has at least one of (a) a higher efficacy than bromochlorodimethylhydantoin against Pseudomonas sp. in water containing 100 mg/l CWC at pH 9.0 after a contact time of 5 hours and (b) a higher efficacy than dibromonitrilopropionamide hours.
 2. A process according to claim 1, wherein said high chlorine demand water is water in the pulp and paper industry.
 3. A process according to claim 1, wherein said sodium hypochlorite is added to a solution of said ammonium bromide.
 4. A process according to claim 1, wherein said water has a chlorine demand of at least 1.8 ppm out of 2.0 ppm Cl₂ after 60 minutes.
 5. A process according to claim 1, wherein said mixture is added to said water at a daily level of at least 2 mg/l as Cl₂.
 6. A process according to claim 1, wherein the temperature of said mixture is 10-30 degrees Celsius.
 7. A process according to claim 1, wherein said mixture is fed continuously to said high chlorine demand water.
 8. A process according to claim 1, wherein said high chlorine demand water is recirculating water.
 9. A process according to claim 1, wherein the temperature of said high chlorine demand water to which said mixture is added is 10-60 degrees Celsius. 